E-UTRA (Evolved Universal Terrestrial Radio Access) (also called LTE (Long Term Evolution) which is a next-generation mobile unit communication standard is established, supposing that a mobile terminal has a plurality of antennas in order to implement MIMO (Multi Input Multi Output). However, as the size of a mobile terminal is reduced, a distance between antennas is extremely reduced in a mobile terminal with a plurality of antennas accordingly. As a result, harmful effects may be caused by a reduction in distance between antennas.
For example, it is assumed that a mobile terminal serving as a radio communication device has two antennas (for example, a first antenna and a second antenna). In the above mentioned case, such a first harmful effect may occur that when a radio wave is to be transmitted via the first antenna, the impedance of a transmission system of the first antenna is made mismatching under the influence of the second antenna and the quantity of reflection components is increased in the first antenna. In addition, such a second harmful effect may occur that part of a signal which has been transmitted via the first antenna is absorbed into (received by) the mobile terminal via the second antenna. Incidentally, a phenomenon that a signal which has been transmitted via a first antenna is absorbed into a mobile terminal via a second antenna is called “coupling” and the quantity of signals which are absorbed via the second antenna is called a “coupling amount”. In the case that a radio wave which has been radiated via the first antenna is absorbed into the mobile terminal via the second antenna or a high-frequency signal is propagated across a substrate of the mobile terminal from the first antenna toward the second antenna, the coupling occurs between the antennas. An inter-antenna coupling amount (an amount of coupling between antennas) tends to be increased with decreasing a distance between antennas.
In the case that the inter-antenna coupling amount is large, such a problem may generate that a desired transmit signal level (in other words, a substantial transmit signal level) is not obtained and hence throughput of communication with a communication device (for example, a base station) as a transmit destination is reduced. If such countermeasures as to increase the amplification degree of an amplifier of a transmission system so as to ensure a desired transmit signal level is taken in order to eliminate the above-mentioned problem, such another problem will generate that the power consumption becomes higher than that used when the inter-antenna coupling amount is small.
In addition, in the case that signals are received via two antennas and transmitted via one of them, when the inter-antenna coupling amount is large, such a problem may generate that the sensitivity to receive a desired wave is relatively reduced.
Nowadays, a radio device (a radio communication device) configured such that a variable coupler is disposed between two antennas so as to cope with coupling which would occur between the antennas thereby to control the coupling amount of the variable coupler using an integrated control device such that the coupling amount is increased in reception and is decreased in transmission is proposed (see, for example, Japanese Laid-open Patent Publication No. 2007-124581). In the above mentioned radio communication device, the variable coupler is controlled in accordance with information on communication states such as an error rate and a receive field intensity of a receive signal.
On the other hand, in the case that diversity reception is to be performed using a plurality of antennas installed on a mobile terminal, in general, it is preferable that a correlation between the antennas relative to a receive signal be low. From the above mentioned viewpoint, a small portable terminal device for radio reception (a radio communication device) which is configured to obtain diversity effect by reducing a correlation between antennas is proposed (see, for example, Japanese Laid-open Patent Publication No. 2007-243455). In the above mentioned radio communication device, a plurality of antenna elements are provided on the tips of a plurality of projecting strips which are formed by notching edge parts of an upper ground conductor. The above mentioned radio communication device includes a variable reactance element and a switch as a correlation changing mechanism for changing a correlation among a plurality of antenna elements. In the above mentioned radio communication device, the correlation changing mechanism reduces the correlation among the antenna elements by changing the phase of a high-frequency current flowing through the upper ground conductor.
Incidentally, the following approximate expression (numerical formula 1) for deriving an inter-antenna correlation coefficient ρe (a coefficient of correlation ρe between antennas) from S parameters (reflection coefficients and transmission coefficients used in a network that handles high-frequency signals) of respective antennas is proposed (see, for example, ELECTRONICS LETTERS, 1st May 2003, Vol. 39 No. 9, “Exact representation of antenna system diversity performance from input parameter description”, S. Blanch, J. Romeu and I. Corbella). Use of the following numerical formula may make it possible to obtain the inter-antenna correlation coefficient from a relatively simple calculation.
                              ρ          e                =                                                                                                                S                    11                    *                                    ⁢                                      S                    12                                                  +                                                      S                    21                    *                                    ⁢                                      S                    22                                                                                      2                                                                                (                                      1                    -                                          (                                                                                                                                                              S                              11                                                                                                            2                                                +                                                                                                                                        S                              21                                                                                                            2                                                                    )                                                        )                                                                                                      (                                      1                    -                                          (                                                                                                                                                              S                              22                                                                                                            2                                                +                                                                                                                                        S                              12                                                                                                            2                                                                    )                                                        )                                                                                        [                  Numerical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
In the above mentioned LTE, a basic communication format that utilizes Frequency Division Duplex (FDD) system is provided for a radio communication device such as a mobile terminal with two antennas. According to the above mentioned communication format, transmission is performed via at least one of a plurality of antennas included in a radio communication device and reception is performed via at least two antennas. In an existing radio communication device, a method of effectively controlling an inter-antenna coupling amount and/or an inter-antenna correlation coefficient so as to cope with a communication format as mentioned above may not be disclosed.
For example, in an existing radio communication device in which a variable coupler is disposed between two antennas, a baseband processing unit calculates information on communication states such as an error rate and a receive filed intensity of a receive signal and the variable coupler is controlled on the basis of the information so calculated. However, an effective information acquiring method and an effective controlling method conforming to the above mentioned communication format may not be disclosed.
Therefore, the present art aims to provide a radio communication devoice and a signal processing method permitting to effectively control an inter-antenna coupling amount and/or an inter-antenna correlation coefficient in the case that a plurality of antennas including transmission and reception antennas are installed to perform communication on the basis of the frequency division duplex (FDD) system.